Digital to analog converter chip

[vuchris]

I've got a PIC18F458 microprocessor feeding an 8 bit number into a digital to analog converter. The analog converter is to spit out -10V for 00000000 and +10V for 11111111, and everything in between. (-10=>0=>+10V)

I'm using a DAC800LCN digital to analog converter chip.

First question:
The data sheet for the chip is located here:
https://www.electro-tech-online.com/custompdfs/2007/10/DAC0800.pdf

On page 3 it has two different types of casing packages. The chip I has the text: JM024AD, DAC0800LCN, DAC-08EP on it, which chip is it, the one on the left or the one on the right? I suspect its the one on the left as the chip has that DOT which indicates where pin 1 is as shown on the figure on the left.

Second question:

I've wired this chip up like this:

**broken link removed**

In the typical application figure 8 labelled "Symmetrical offset binary operation" which is the mode of operation I'm after. If I wire up Vref(+) to +10V, Vref(-) to -10V and the rails at +/-15V, as shown in my drawing, do I need that op amp out the front? Will Io spit out -10 to +10V depending on the binary input? (00000000=-10V, 11111111=+10V).

If thats the case, if you look at the typical application noted on page 1, figure 1, why have they got the resistors on the output to what appears to be Vref(+). Do I need these resistors? ie. is the chip open collector? or can I just take the voltage (+/-10V, 20vp-p) out of Io without any external resistances in place.

Your help is appreciated.

Thank you

 

[vuchris]

*bump* anyone?

 

[ericgibbs]

First question:

On page 3 it has two different types of casing packages. The chip I has the text: JM024AD, DAC0800LCN, DAC-08EP on it, which chip is it, the one on the left or the one on the right? I suspect its the one on the left as the chip has that DOT which indicates where pin 1 is as shown on the figure on the left.
The right side is a surface mount device, the pins are bent at right angles to the chip, it appears you have the DIP dual in line package, pins thru the pcb or a socket.


Second question:

I've wired this chip up like this:

**broken link removed**

In the typical application figure 8 labelled "Symmetrical offset binary operation" which is the mode of operation I'm after. If I wire up Vref(+) to +10V, Vref(-) to -10V and the rails at +/-15V, as shown in my drawing, do I need that op amp out the front? Will Io spit out -10 to +10V depending on the binary input? (00000000=-10V, 11111111=+10V).

If thats the case, if you look at the typical application noted on page 1, figure 1, why have they got the resistors on the output to what appears to be Vref(+). Do I need these resistors? ie. is the chip open collector? or can I just take the voltage (+/-10V, 20vp-p) out of Io without any external resistances in place.

Your help is appreciated.

Thank you

hi,
I'll refresh my thoughts on the DAC in your circuit and get back to you.

 

[ericgibbs]

hi,
The outputs from the DAC are 'currents', in order to generate a 'voltage' from this current a load resistor is required.

In the page1 application, 10K resistors are used in figure 8 the OPA is the equivalent 'load resistor'.

The advantage of the OPA is that you will get a voltage output swing from the OPA of +/-10V, which is what you require,
rather than two outputs using the 10K resistors.

I would recommend a more 'modern' OPA that the 741 shown, a LM358 or equivalent would be much better.

Ensure that the +/-Vrefs are stable, else you could get drift problems.


Does this explain OK?

 

[vuchris]

Well I got it working. I wired up the 10k resistors on the output to a seperate +10V source. However, now Vref(+) has no effect on the output max voltage, only Vref(-). Theyre both sitting at +/- 13V, but I cant get the otuput to go past 9.64V.

 

[vuchris]

never mind, got it working.

 

[ericgibbs]

never mind, got it working.

Just beat me to it!
What did you do to get it working, for future reference?

This WAS my suggestion.
hi,
Disconnect the -Vref to -10v, connect a 5K resistor from -Vr pin15 to 0V
Connect a 5K resistor from +Vr pin14 to the +10Vr, as shown on page 7 of the datasheet.

Try it and lets know what output swing you get?